With increasing energy consumption, the world is speeding up the development of an alternative energy source that is eco-friendly and offers unlimited resources in replace of traditional energy sources, such as coal, oil, nuclear, and the like, that are exhaustible and have environmental issues.
Such alternative energy sources include solar, wind, wave, geo-thermal, and the like, and various studies are being made to apply these new energy sources in real life. Among them, solar power is the most representative alternative energy source used in daily life, and has widespread applications ranging from a home solar power generator to a solar cell light fixture.
In particular, a solar cell light fixture is an application of a solar power generation system that converts solar energy to electrical energy using a solar cell, stores the electrical energy converted from the solar energy in a secondary battery during the day, and uses the stored electrical energy as a power source of the solar cell lighting apparatus at night. The solar cell light fixture is gaining popularity for its easy installation, low installation and maintenance costs, and no need for electrical cable laying, in areas difficult to bury and wire electrical lines such as seashores, mountaintop, farmlands, hiking trails, and the like, areas requiring special considerations for environmental protection such as tourist spots, theme parks or amusement parks, research facilities, and the like, or areas requiring aesthetic considerations such as parks, walking trails, gardens, grave sites, and the like. Examples of a solar cell light fixture include a landscape light, a street light, a security light, and the like.
However, since a solar cell light fixture is designed to obtain power from solar energy, it is incapable of power charging during the night, rain/snow, and gloomy days when the sun doesn't shine. Therefore, a dimming control based on a charge capacity of a secondary battery is necessary. Further, a charge capacity of a secondary battery may be different for each solar cell light fixture. For example, if solar cell light fixtures are installed in an area near a building, an amount of energy stored in a secondary battery may be different for each solar cell light fixture due to shadow casting by the tall buildings. Accordingly, to achieve a uniform charge capacity, balancing the charge capacities of the secondary batteries by calculating an operation time of each of the solar cell light fixtures is needed.
There are many methods for balancing charge capacity of secondary batteries. A typical example is a method that adds a boost circuit or a buck circuit for each secondary battery, and charges a secondary battery having a relatively low charge capacity or discharges a secondary battery having a relatively high charge capacity.
The charge capacity balancing method of adding the buck circuit has advantages of being simple to configure the buck circuit including a resistance element, and being cost efficient and easy to control. However, a disadvantage is that energy stored in a secondary battery having a relatively high charge capacity is discarded through discharging rather than being used to achieve charge capacity balancing. In contrast, the charge capacity balancing method of adding the boost circuit is advantageous in that balancing is accomplished at a generally high level of charge since the charge of a secondary battery having a relatively low charge capacity may increase. However, there are drawbacks in that the boost circuit is more expensive than the buck circuit and is difficult to control. Accordingly, an appropriate secondary battery balancing method for a solar cell light fixture is required.